During neurological surgery, therapy or diagnosis, it is common practice to work with a reference system being fixed in relation to the head of the patient using some type of fixation device. The reference system is used to determine the location of different areas in the involved part/tissue of the patient.
An example of such a reference system is a so-called stereotactic frame provided with pin support members in form of posts having fixation pins for invasive fixation to the skull of a patient. In use during for example MRI (Magnetic Resonance Imaging) diagnostics, the stereotactic frame is arranged around the head of a patient, and the fixation pins of the posts connected to the frame are screwed into or to abutment against the bone of the skull, thus ensuring a rigid fixation of the reference system. The frame is then rigidly held in position in relation to a MRI table.
The PCT-application SE 03/00723 discloses a fixation device comprising a pin support member. Besides a pin support member and a fixation pin, this fixation device comprises a sleeve shaped insulation means which is arranged to be supported on the pin support member and at least partly surround and hold the fixation pin. This is done to restrict the electrical coupling between the pin support member and the fixation pin when those are made of electrically conductive materials, and thus prevent temperature rise in the pins and the areas around them, such as the tissue of the head, due to the electrical coupling.
A problem associated with prior art fixation devices is that the fixation pins in most cases protrude a distance beyond the outer boundaries of the pin support members. This is sometimes due to the fact that the fixation pins are provided with external engagement means on their outer ends, to be engaged when threading them to or from a fixation position, which preclude completely screwing in of the fixation pins into the pin support members. Also in the case where the fixation pin is provided with internal engagement means on its outer end, it is common that the fixation pin extends through the entire pin support member and protrudes on the outer side of the pin support member, due to the need for sufficient support for the fixation pin from the pin support member and the absence of fixation pins having correctly adapted length. Such protruding ends of the fixation pins may interfere with the equipment and apparatuses used during the diagnosis, therapy or surgery.
Moreover, the frame members used are usually provided in only one size to reduce the costs, and designed to be able to receive the largest expected size of head. This has to result that when fixation of a small and a more common medium sized head, there will be needed exceedingly long fixation pins to be able to secure the head. Long fixation pins may be desirable also when displacing the head in relation to the reference system in order to reach peripheral disposed target areas in the head.
The use of the long fixation pins for holding the weight of the head, give rise to a large bending moment, which has to effect that the deflection, of even high quality fixation pins, will be comparatively large. The fixation thus becomes unstable with an obvious risk that the target area for treatment may become considerably displaced in relation to the reference system from the point of time when generating an image of the target area until treatment.
As a rule it is namely common practice that an image will in a first step be generated of the target area. During analysis of the image and deciding upon the best way for treatment, the patient will be disconnected from the image generating apparatus, which normally is a MRI or a CT but which also may be a X-ray, PET or MEG apparatus, but will still have the fixation device secured to the head by means of the fixation pins. When the treatment are to begin, the patient will be connected to a different equipment for surgery, radiation or other type of treatment, and it is then extremely important that the head will be secured in exactly the same position in relation to the reference system and the equipment as when generating the image, in order to achieve effective treatment of the relevant parts of the brain and prevent treatment of healthy parts.
Different load conditions, which can lead to displacement of the head in relation to the reference system, may occur between the image generating occasion, which in most cases is performed with the patient lying on the back, and the occasion of treatment when the patient may be placed in a different position, e.g. sitting. In most cases a divergence of less than about 0.1 mm is desirable.
To allow fixation of heads of all different sizes, it is also necessary to provide fixation pins in many different lengths. It is not unusual that twenty or more different lengths of fixation pins have to be provided. Of course this makes the fixation device more expensive to produce and more intricate to use.